1. Field of the Invention
The present invention generally relates to the field of scanning probe microscopes and, more particularly, relates to an actuating and sensing device for scanning probe microscopes.
2. Description of Related Art
Microfabricated cantilever beams including a probing tip attached thereto are one of the main components of scanning probe microscopes (SPM), for example, atomic force microscopes (AFM). A measuring mode of an SPM is the so-called “dynamic” mode. In this mode, a tip is brought very close to a sample surface and the cantilever is vibrated with a frequency, which is close to its resonance frequency. In different measuring modes, for example so called “tapping” or intermittent contact modes, the tip is allowed to touch the surface as the cantilever vibrates. While a sample is scanned, the distance between the tip and features of the sample surface vary. This variation causes changes in the gradient of interaction forces, for example van der Waals forces, between tip and surface. The resulting changes in the mechanical characteristics of the cantilever, such as the resonance frequency, phase, vibration amplitude, and Q-factor, are detected with external systems, for example optical deflection detection systems. Usually, the distance between cantilever and sample surface is controlled by a feedback system to maintain the characteristics property at a constant value.
Cantilevers are typically vibrated by using a piezo slab attached to a cantilever chip. Spring constants k of conventional cantilever beams used in the tapping mode are usually k=1–100 N/m with resonance frequencies of 5–300 kHz. Low spring constant cantilevers are preferred because in that way the tip is less damaged or worn during operation. High resonance frequency cantilevers are preferred for high throughput or speed SPM measurements. As an example, it is difficult to use very soft cantilevers for dynamic mode measurements in air, e.g. with a spring constant with a value less than 0.1 N/m, if there is no sufficient vibrating amplitude provided, e.g. up to 1 μm. For example, water on a sample surface traps the tip to the surface without releasing it again. A piezo slab attached to the cantilever cannot effectively provide the tip with a sufficient excitation if the cantilever is vibrated at a frequency different from the first resonance frequency of the cantilever. The frequency of such a system, piezo slab and cantilever chip, is also not effective at higher frequencies than the first resonance frequency of the system.
In other implementations of SPMs, quartz tuning forks are used instead of microfabricated cantilevers. Tuning forks are electrical components that were mainly developed for electronic circuits. They are small mechanical resonators of a few mm in size and have a very high Q-factor, i.e. they are very sensitive to applied forces. The relatively easy accessibility to their resonance characteristics, for example by measuring the electrical conductance, make tuning forks attractive candidates for SPM applications. In SPM applications using a tuning fork, an SPM tip is attached to one prong of the tuning fork. The tip is attached either sideways or on top of the prong as disclosed, for example, in the documents U.S. Pat. No. 6,094,971 and EP 0 864 846. The disadvantages of such cantilever systems arise from the fact that the tips are fixed directly to one prong of the tuning fork. The symmetry of the tuning fork is broken. This reduces the mechanical Q-factor and makes it less sensitive to applied forces. Further, the vibration amplitude of the tip is always the same as that of the tuning fork itself. In addition, these probes are very stiff compared to conventional microfabricated cantilevers, i.e. tips are easily damaged during operation. Typical spring constants of prongs of tuning fork resonators are 1.8 kN/m with a resonance frequency of about 30 kHz.